1. Field of the Invention
This invention is related to the fabrication of truss or frame structures, such as those used in an aircraft fuselage and to structural connectors that can be used not only to connect tubular members forming the structure, but which also serve as jigs for assembling the components of the structure.
2. Description of the Prior Art
There are a number of methods of fabricating an aircraft structure, in particular a fuselage structure suitable for use in a light aircraft. One approach employs a structural frame or truss to support all or substantially all of the loads or forces that must be carried by the fuselage. Another approach, commonly employed on larger or more sophisticated aircraft is to employ a fuselage constructed of thin sheets or webs of sheet metal. The sheets are suitable for resisting shear or tension loads in the plane of the sheets. These sheets must be stiffened by members more capable of carrying compression loads and loads normal to the sheet, or skin or web. Semimonocoque structures employ thin webs, such as the skin or a fuselage, to carry tension and shearing forces and stiffeners to carry compression or normal loads. A semimonocoque fuselage structure typically employs closely spaced rings or bulkheads, which resist loads in transverse planes, while the fuselage shell resists loads in the longitudinal direction. Additional longitudinal structural members, such a stiffeners, stringers or longerons span between bulkheads and transfer loads to the bulkheads.
The simpler trusses or frames commonly employed in light aircraft commonly employ chrome-molybdenum steel tubes. Tubular frame structures formed from welded chrome-molybdenum tubes are the standard structural components used in light and ultralight aircraft. These tubular frame structures or trusses are commonly employed with a fabric or non-load bearing outer surface or external skin. They also require extensive bracing and cross bracing.
Welding is used extensively for steel-tube truss structures, such as fuselages. The most common type of welding consists of heating parts to be joined by means of an oxyacetylene torch and then fusing them together with a welding rod. The tensile strength at the weld can become similar to that of cast metal, and it is more brittle and less able to resist shock and vibration loading than is the original material. Aircraft tube walls are thin and more difficult to weld than other machine and structural members. At one time all aircraft welding was torch welding, but electric arc-welding has also been used. For arc-welding, the welding rod forms an electrode from which current passes in an arc to the parts being joined. The electric arc simultaneously heats the parts and deposits weld metal from the electrode. Heating is more localized than for torch welding, and the strength of the heat-treated parts is not impaired as much by arc-welding.
The strength of conventional welded joints depends largely on the skill of the welder. The stress concentrations can vary and it is customary to design welded joints for aircraft fuselages with a liberal margin of safety. Welded joints should be in shear or compression but design often dictates that tensile loads must be applied to a welded joint. Steel tubes, such as chrome-molybdenum alloy tubes, are usually spliced by prior art fish mount joints as shown in FIG. 17. These joints are designed so that most of the weld is in shear and so that most of the weld is not confined to one cross section of the tube. If a butt weld in necessary, the weld should be diagonal and not perpendicular to the centerline of the tube, as shown in the prior art weld of FIG. 18.
Fuselage truss members are often welded as shown in the prior art weld shown in FIG. 19. In that Figure only the horizontal member is highly stressed. If members other than the horizontal member are stressed, common prior art practice is to insert gusset plates as shown in FIG. 20. Steel tubes often have walls as thin as 0.035 in. The welder must control the temperature to keep from overheating the thin walls and burning holes in them. It is extremely difficult to weld a thin member to a heavy one, as more heat is required for the heavy member. The thickness ratio of parts being welded should be less that 3:1, and preferably less than 2:1.
Conventional concentric butt welded fuselage joints between tubes in aircraft and fuselage structure may be satisfactory where vibration is not present. However, the fatigue strength of butt welded joints is compromised when subject to reverse bending. Therefore common practice requires that finger plates or insert gussets should be added to joints subject to vibration. Indeed, the standard practice used in fabricating light and ultralight aircraft is to weld gusset plates at welded intersections of tubes in the fuselage and cabin. However, the configuration of the different welded joints in an aircraft fuselage is generally not uniform. This lack of uniformity gives rise to two problems. First that shape of the tubular members at different joints will be different, in part because of the orientation of the tubular members entering that joint, and the shape of the gusset plates will also differ from joint. This means that a large number of different parts are necessary and that jigs are necessary both for the fabrication of different components as well as for the assembly of multiple components at each joint. The integrity of the welded structure is also dependent upon the skill of the welder, and each weld can take a relatively large amount of time to complete.
Another approach to connecting thin-wall hollow tubes to create a lightweight three dimensional truss structure that can be used in aircraft is shown in U.S. Pat. No. 4,624,599. According to the method disclosed in that patent, the ends of coplanar tubes are partially flattened into an elongated flattened oval shape. Portions of the ends of the tubes are cut away so that the oval ends can be partially telescoped to fit in a mutually nesting relationship with partially flattened ends overlapping. Multi-layer sandwich splice plates are located on the interior of the oval end sections and the plates are bolted to the flattened ends of the tubes so that the tubes can be clamped together. An overlying bracket including formed end plates and welded gussets is used to connect tubes extending in different planes. It would appear that each of these joints would require considerable fabrication and assembly. Not only are the tube ends to be deformed into an oval shape, but the ends of the tubes are machined so that the tube ends can partially telescope at a prescribed angle. Each splice plate is also formed from multiple components which must be separately machined and assembled. Not all joints in an aircraft structure, such as a fuselage, have the same configuration, so it would appear to be necessary to separately machine, form and fabricate and assemble different subcomponents at each joint, and fabricate multiple dies for different joint components. As such, this approach would appear to be a rather expensive way to fabricate a light aircraft.
Structural frames, such as frames forming an aircraft fuselage or cabin, fabricated using these prior art techniques tend to be labor intensive to assembly, especially when gussets must be welded to the tubular members, and when the shape of the components, such as gusset plates, must be different for virtually all nodes of the frame. A large number of different parts are required and the quality and integrity of welds are often dependent on the skill of the welder. Care must also be taken to insure that the load carrying capacity of the tubular members is not diminished by the welds and that appropriate safety margins are not compromised. It is also important that the overall weight of the structure does not become too great. The instant invention, comprising a method of assembling and welding a structural frame, such as an aircraft fuselage, and components employed in the frame, addresses these problems. This invention also provides a simple and relatively inexpensive means to fabricate a light aircraft structure.
A method of assembling an aircraft fuselage, according to this invention, includes the following steps. Connector blocks are formed with bore holes extending from edges and through holes extending between opposite faces. Ends of a first set of lateral tubes are inserted into bore holes in connector blocks. The bore holes in individual connector blocks are oriented to function as jigs to properly orient the tubes to form a portion of an aircraft fuselage frame. A second set of longitudinal tubes are inserted into the through holes so that the tubes in the second set extend continuously through the connector blocks. The tubes in the second set form longitudinal members in the aircraft fuselage. The through holes are oriented to function as jigs to properly orient longitudinal members relative to each other and relative to the remainder of the aircraft fuselage.
More specifically the method of fabricating an aircraft fuselage according to this invention includes the step of assembling a series of triangular bulkheads. Each bulkhead has three tubular members assembled to connector blocks at the apices of each triangular bulkhead. Each connector block has two bore holes extending into a peripheral edge of the connector block. Each triangular bulkhead is assembled by inserting opposite ends of each of the three tubular members into bore holes in the connector blocks. Longerons are inserted into through holes on the connector blocks so that a series of triangular bulkheads are positioned at spaced longitudinal positions on the longerons to form an internal frame for the aircraft fuselage. An external skin can be added attached to the connector blocks to form a double fuselage with both the truss or frame and the outer skin carrying loads or forces applied to the aircraft fuselage. The tubes can also be welded to the connector blocks.
The assembly constructed according to these methods includes tubular members and structural connectors for use in connecting the tubular members to form a structural frame, such as an aircraft fuselage frame or truss. The structural connectors comprise a block having a thickness greater than an external diameter of tubular members to be connected by the corresponding structural connector. At least one bore hole extends into the block from a peripheral edge of the block. The peripheral edge extends between opposite faces of the block. The bore hole has an inner diameter sufficient for insertion of an end of a tubular member into the bore hole. A through hole extends between the opposite faces of the block. The through hole has a diameter sufficient to permit a second one of the tubular members to pass through the block. The block connects at least two tubular members to form a portion of the structural frame or fuselage. The tubes can be plug welded to the structural block, and the bore holes and through holes function as jigging means for properly assembling the tubular members.
These structural connectors can also include a transverse hole extending from one face of the block and intersecting a bore hole, that extends inwardly beyond the transverse hole. The transverse hole permits an assembler to determine if the end of the tubular section is fully inserted into the bore hole. The transverse hole can also provide an opening for plug welding the structural connector to the tubes.
An aircraft frame using these structural connector blocks and constructed according to this method has a plurality of tubular sections connected by a plurality of connector blocks and an external skin. The tubular sections comprise both longerons and lateral tubular sections. The connector blocks connect a plurality of lateral tubular sections to each longeron at multiple longitudinal positions on each longeron. The external skin is attached to the connector blocks so that loads, applied to the aircraft frame, are carried by both the tubular sections and the external skin. Lateral tubular sections or members form bulkheads in which the tubular members are joined together at the ends thereof by the connecting blocks located at apices of each bulkhead. The connecting blocks also connecting the bulkheads to the longerons.